In a typical polymerisation reaction, monomer, diluent, catalyst, co-catalyst and optionally co-monomer and hydrogen are fed to a reactor where the monomer is polymerised. The diluent does not react but is typically utilised to control solids concentration and also to provide a convenient mechanism for introducing the catalyst into the reactor. The reactor effluent, a mixture of polymer, diluent, unreacted (co-)monomer and hydrogen, is removed from the reactor and fed to a flash tank where the polymer is separated from the diluent and unreacted (co-)monomer and hydrogen. Typically, catalyst will be contained in the polymer.
Polymerisation processes of ethylene may be carried out in loop reactors. In the polymerisation reaction of ethylene, different reactants including the monomer ethylene, a light hydrocarbon diluent such as isobutane, a catalyst and optionally, a co-monomer such as hexene-1 and hydrogen are fed to a reactor. When polymerising ethylene, in the presence of a suspension of catalyst in diluent, said diluent having low solubility for the polymer, the polymer is produced in the form of solid particles, insoluble in the diluent. The contents of the reactor are circulated continuously with a pump to avoid deposition of polymer on the walls of the reactor. Slurry, consisting of the reactants and polyethylene powder, is typically collected in one or more settlings legs of the polymerisation reactor and discharged continuously to a flash tank, through flash lines, where most of the light hydrocarbon diluent and unreacted ethylene evaporates, yielding a dry bed of polyethylene in powder form. The powder is discharged to a purge drier in which the remaining light hydrocarbon and co-monomer are removed. Then the powder of polyethylene is transported to a finishing area where various stabilisers and additives are incorporated. Finally it is extruded into pellets.
For obtaining polymer having suitable properties, it is essential in a polymerisation reaction to control the reaction conditions and input component quantities in the reactor. For doing so, it is conventional to sample the reactor contents and control several of the variables of the process in response with the analysis of the sample.
Several methods have been described to take samples from the reactor contents. Generally the reactants in loop-type reactors are propelled at relatively high velocities in order to maintain the catalyst and particulate polymer produced in a suspended state and to prevent deposition or growing of polymer on the reactor walls. It is therefore necessary that no vapor phase is present in the reactor where polymer might grow. In order to take a sample from such reactors, generally a standpipe is placed in the uppermost portion of the reactor to collect slurry. However, the slurry in said standpipe is generally not in equilibrium with the reactants, and hence it is almost entirely impossible to obtain a representative sample.
A vapor sample may be taken from the flash tank. However, sampling of gases from flash tanks has several disadvantages. In polymerisation plants using flash tanks which are connected to a reactor by means of flash lines and settling legs, the settling legs themselves can present problems. Conventional settling legs have sections in which polymer can collect while waiting for next dump cycle for transferring the slurry to a flash tank. The collected polymer can melt over time and deposit on the inside walls of the settling leg. In addition, during collection of the slurry in the settling legs and before dumping it to the flash tank, the polymerisation reaction still continues. Also, there is a lag in time between recovery of slurry in the settling legs and further processing of the slurry to the flash tank. As a consequence thereof, reaction conditions, which are monitored after transfer of the slurry in the flash tank, are different from the reaction conditions in the reactor. Analysis of a gas sample taken from the flash tank does not provide updated information on the reaction conditions in the polymerisation reactor and will result in an inaccurate analysis of the gas composition in the polymerisation reactor.
U.S. Pat. No. 3,556,730 refers to a sampling apparatus for taking a sample comprising liquid, dissolved gas and suspended particulate solids from a reactor into a fixed volume chamber. The reaction fluid in the chamber is then rendered non-reactive by immediately adding a predetermined volume of reaction termination fluid. The non-reactive sample is automatically discharged into a separation chamber from which part of the dissolved gas and liquid is continuously analysed.
U.S. Pat. No. 6,042,790 describes an apparatus and method for maintaining unreacted monomer concentration in a polymerisation reactor. In a polymerisation process utilising a high pressure flash to separate polymer from unreacted monomer contained in the effluent stream from the reactor, the concentration of unreacted monomer in the reaction effluent is determined by withdrawing from the reactor an effective analysing amount of effluent, exposing the amount to a low pressure flash and analysing the vaporised portion to determine the concentration of monomer. However, the described apparatus and method do not allow to take solid particles out of the reactor and to analyse these.
U.S. Pat. No. 4,469,853 provides a method for preparing polyolefins. A step in this method consists of detecting the concentrations of olefin and hydrogen in the gas phase within the reactor by gas chromatography. However, the described apparatus and method do not allow to take solid particles out of the reactor and to analyse these.
U.S. Pat. No. 6,037,184 discloses a method and apparatus for taking a sample out of a flowing suspension formed by polymer particles and hydrocarbon diluent in a olefin polymerization process. The apparatus relies on the use of a filter which is placed either straight to the wall of the loop reactor, whereby a sample is taken straight from the loop reactor, or in a transfer pipe which connects two loop reactors. A sample is taken out of the reactor and transferred to a vaporizing pressure reducer. The vaporized sample is then introduced in a wax separation vessel and further in a wax removal vessel. However, the sample taken out of the loop reactor does not contain solid particles. Analysis of said sample is therefore not fully representative for the reaction conditions in the loop reactor. In addition, in embodiments wherein a sampling apparatus is provided in a transfer pipe which connects two loop reactors, reaction conditions, which are monitored after transfer of the slurry in the transfer pipe, are different from the reaction conditions in the loop reactor. Thus, analysis of a sample taken from such transfer pipe does not provide fully updated information on the reaction conditions in the polymerisation reactor.
A drawback in the above-described devices and methods is that they do not allow the control of several different variables of the polymerisation process, such as e.g. monomer, co-monomer and hydrogen in the gas phase and properties of the polymerisation product such as the melt flow index and density, in response with the analysis of the sample.
In addition, the above-described methods and devices are not suitable for controlling the polymerisation reaction in system wherein bimodal polyethylene is prepared, i.e. in system comprising two interconnected polymerization reactors. In particular, the above-described methods and devices do not provide for a representative sampling of a first polymerisation reactor in such bimodal system.
In view hereof, it is clear that there remains a need in the art for providing a more accurate sampling system for taking and analysing a sample from a polymerisation reactor. It is therefore an object of the present invention to provide a device capable of taking out a sample from a polymerisation reactor and accurately analysing said sample. It is further an object of the invention to provide a device capable of taking out a sample from a polymerisation reactor, which consists of two reactors being connected in series. Another object of the invention is to provide a sampling system for taking and analysing a sample from a polymerisation reactor wherein the solid as well as the gaseous phase of said sample are analysed.
It is another object of the present invention to provide a method for improving a polymerisation reaction in a polymerisation reactor. In particular, the invention aims to provide a method for improving a polymerisation reaction for preparing bimodal polyethylene in a polymerisation reactor, which consists of two reactors being connected in series.